Impaling Crystal Skill has abnormal behavior that makes it impossible to run melee dps in this fight or the boss's targeting script is what has issues.
It reads "Deals lightning damage to the current target" which would be perfectly fine if it chose the first target that walks up to the boss to deal this damage to, considering that it doesn't say it will attack lowest hp target.
It doesn't matter where I place my melee dps, the skill will always land on the melee dps and not the tank.
Video attached.
Another possible reason here is that the boss is re-selecting his target to lowest HP target after using his AoE attack. Either way, that isn't what the skill says.